In 1821, the English chemist Sir Humphrey Davy discovered that all metals have a positive temperature coefficient of resistance. This discovery led to the development of the resistance temperature detector (RTD), which today is a widely used device for the measurement of temperature in industrial processes.
The quantum mechanical explanation for this effect is that the metal nuclei vibrate with an amplitude that depends on the temperature of the metal. These nuclear vibrations generate phonons within the metal, the energy of which depend on the amplitude of the nuclear vibrations. When electrons flow through the metal, they tend to be scattered by these phonons. This interaction, which disrupts the smooth flow of electrons through the metal, is what we measure as resistivity. As the temperature of the metal increases, the nuclear vibrations increase in amplitude and generate higher energy phonons. These higher-energy phonons scatter electrons more effectively, thereby increasing the resistivity of the metal.
An RTD is typically made by wrapping a length of metal wire around a ceramic bobbin, or by depositing a thin film of metal on a substrate. Generally, a metal having high resistivity is used, so as to minimize the amount of metal required. Because of its resistance to contamination and its stable and predictable temperature coefficient, a commonly used metal is platinum.
To measure temperature with an RTD, one exposes the RTD at the site whose temperature is of interest and allows the RTD to reach thermal equilibrium. One then passes a known current through the RTD. Preferably, this current is relatively small to minimize measurement error arising from ohmic heating of the metal in the RTD. One then measures the voltage across the RTD. From the known current and the measured voltage, one can calculate a resistance whose value is indicative of temperature.
In practice, the RTD is often physically inaccessible. For example, the RTD might be placed deep in a caustic chemical bath, remote from the measurement instrumentation. As a result, extended wire leads are generally required to connect the RTD to a voltmeter. In such cases, the resistance measured by the voltmeter is the sum of the RTD resistance and the lead resistance associated with the extended wire leads. This lead resistance introduces error in the measurement.
To the extent that the lead resistance is much smaller than the RTD resistance, the fact that the RTD resistance measurement is corrupted by the lead resistance results in only a small error. However, in most RTDs, even a small change in the RTD resistance translates to a significant change in temperature. For example, in the case of a platinum RTD, the resistance is 100 ohms at 0 degrees C. and changes by 0.00385 ohms per ohm degree C. Hence, a 100-ohm RTD changes its resistance by only 0.385 ohms per degree of change in temperature. Thus, a lead resistance as small as two ohms results in a five degree C. measurement error. Given that the leads to the RTD in many industrial applications can be as much as half a mile long, it is easy to see how the lead resistance can significantly reduce the accuracy of the temperature measurement.
To avoid this difficulty, it is known to provide a first pair of leads extending from the current source to the RTD and a second pair of leads extending from the RTD to the voltmeter. In this configuration, referred to as the four-wire RTD interface, the lead resistance between the voltmeter and the RTD does not introduce an error because there is no current in those leads. Although the four-wire RTD interface eliminates the effect of lead resistance, it does so at the cost of doubling the length of wire required.
Another known method of eliminating the effect of lead resistance is the three-wire RTD interface. This interface uses a first current source connected to a first terminal of the RTD by a first lead and a second, identical current source connected to the second terminal of the RTD by a second lead. A third lead connected to the second terminal of the RTD provides a return path for the current provided by both current sources. In the three-wire RTD interface, one can subtract the voltage measured at the second lead from the voltage measured at the first lead to obtain the voltage across the RTD. However, the accuracy of the three-wire RTD interface relies heavily on the two current sources being identical. As a practical matter, it is difficult to provide two current sources that perform identically.
It is thus an object of the invention to provide an interface for measurement of temperature with an RTD that eliminates the effect of lead resistance without using excessive lengths of wire and without relying on two identical current sources for accurate temperature measurement.